The present invention relates to thermosetting compositions of one or more capped polyisocyanate crosslinking agents and one or more hydroxy functional polymers. The hydroxy functional polymer is prepared by atom transfer radical polymerization, and has well defined polymer chain structure, molecular weight and molecular weight distribution. The present invention also relates to methods of coating a substrate, substrates coated by such methods, and composite coating compositions.
Reducing the environmental impact of coatings compositions, in particular that associated with emissions into the air of volatile organics during their use, has been an area of ongoing investigation and development in recent years. Accordingly, interest in powder coatings has been increasing due, in part, to their inherently low volatile organic content (VOC), which significantly reduces air emissions during the application process. While both thermoplastic and thermoset powder coatings compositions are commercially available, thermoset powder coatings are typically more desirable because of their superior physical properties, e.g., hardness and solvent resistance.
Low VOC coatings are particularly desirable in the automotive original equipment manufacture (OEM) market, due to the relatively large volume of coatings that are used. However, in addition to the requirement of low VOC levels, automotive manufactures have very strict performance requirements of the coatings that are used. For example, automotive OEM clear top coats are typically required to have a combination of good exterior durability, acid etch and water spot resistance, and excellent gloss and appearance. While liquid top coats containing, for example, capped polyisocyanate and polyol components, can provide such properties, they have the undesirable draw back of higher VOC levels relative to powder coatings, which have essentially zero VOC levels.
Powder coatings compositions containing polyol and capped polyisocyanate components (xe2x80x9cisocyanate cured powder coatingsxe2x80x9d) are known and have been developed for use in a number of applications, such as industrial and automotive OEM topcoats. Such isocyanate cured powder coatings compositions are described in, for example, U.S. Pat. Nos. 4,997,900, 5,439,896, 5,508,337, 5,554,692 and 5,777,061. However, their use has been limited due to deficiencies in, for example, flow, appearance and storage stability. Isocyanate cured powder coating compositions typically comprise a crosslinker having two or more capped isocyanate groups, e.g., a trimer of 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane capped with e-caprolactam, and a hydroxy functional polymer, e.g., an acrylic copolymer prepared in part from hydroxyalkyl (meth)acrylate. The hydroxy functional acrylic polymers used in such isocyanate based powder coatings compositions are typically prepared by standard, i.e., non-living, radical polymerization methods, which provide little control over molecular weight, molecular weight distribution and polymer chain structure.
The physical properties, e.g., glass transition temperature (Tg) and melt viscosity, of a given polymer can be directly related to its molecular weight. Higher molecular weights are typically associated with, for example, higher Tg values and melt viscosities. The physical properties of a polymer having a broad molecular weight distribution, e.g., having a polydispersity index (PDI) in excess of 2.0 or 2.5, can be characterized as an average of the individual physical properties of and indeterminate interactions between the various polymeric species that comprise it. As such, the physical properties of polymers having broad molecular weight distributions can be variable and hard to control.
The polymer chain structure, or architecture, of a copolymer can be described as the sequence of monomer residues along the polymer back bone or chain. For example, a hydroxy functional copolymer prepared by standard radical polymerization techniques will contain a mixture of polymer molecules having varying individual hydroxy equivalent weights. Some of these polymer molecules can actually be free of hydroxy functionality. In a thermosetting composition, the formation of a three dimensional crosslinked network is dependent upon the functional equivalent weight as well as the architecture of the individual polymer molecules that comprise it. Polymer molecules having little or no reactive functionality (or having functional groups that are unlikely to participate in crosslinking reactions due to their location along the polymer chain) will contribute little or nothing to the formation of the three dimensional crosslink network, resulting in less than desirable physical properties of the finally formed polymerizate, e.g., a cured or thermoset coating.
The continued development of new and improved isocyanate cured powder coatings compositions having essentially zero VOC levels and a combination of favorable performance properties is desirable. In particular, it would be desirable to develop isocyanate cured powder coatings compositions that comprise hydroxy functional polymers having well defined molecular weights and polymer chain structure, and narrow molecular weight distributions, e.g., PDI values less than 2.5. Controlling the architecture and polydispersity of the hydroxy functional polymer is desirable in that it enables one to achieve higher Tg""s and lower melt viscosities than comparable hydroxy functional polymers prepared by conventional processes, resulting in thermosetting particulate compositions which are resistant to caking and have improved physical properties.
International patent publication WO 97/18247 and U.S. Pat. Nos. 5,763,548 and 5,789,487 describe a radical polymerization process referred to as atom transfer radical polymerization (ATRP). The ATRP process is described as being a living radical polymerization that results in the formation of (co)polymers having predictable molecular weight and molecular weight distribution. The ATRP process is also described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.). The ""548 and ""487 patents and WO 97/18247 patent publication also describe (co)polymers prepared by ATRP, which are useful in a wide variety of applications, for example, with paints and coatings.
In accordance with the present invention there is provided, a thermosetting composition comprising a coreactable solid, particulate mixture of:
(a) capped polyisocyanate crosslinking agent; and
(b) hydroxy functional polymer prepared by atom transfer radical polymerization initiated in the presence of an initiator having at least one radically transferable group, and in which said polymer contains at least one of the following polymer chain structures I and II:
xe2x80x94[(M)pxe2x80x94(G)q]xxe2x80x94xe2x80x83xe2x80x83I
and
xe2x80x94[(G)qxe2x80x94(M)p]xxe2x80x94xe2x80x83xe2x80x83II
wherein M is a residue, that is free of hydroxy functionality, of at least one ethylenically unsaturated radically polymerizable monomer;. G is a residue, that has hydroxy functionality, of at least one ethylenically unsaturated radically polymerizable monomer; p and q represent average numbers of residues occurring in a block of residues in each polymer chain structure; and p, q and x are each individually selected for each structure such that said active hydrogen functional polymer has a number average molecular weight of at least 250.
In accordance with the present invention, there is also provided a method of coating a substrate with the above described thermosetting composition.
There is further provided, in accordance with the present invention, a multi-component composite coating composition comprising a base coat deposited from a pigmented film-forming composition, and a transparent top coat applied over the base coat. The transparent top coat comprises the above described thermosetting composition.
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as modified in all instances by the term xe2x80x9cabout.xe2x80x9d
As used herein, the term xe2x80x9cpolymerxe2x80x9d is meant to refer to both homopolymers, i.e., polymers made from a single monomer species, and copolymers, i.e., polymers made from two or more monomer species.